The present invention relates generally to steering on tractors and more particularly to achieving a small turning radius on an axle oscillatable tractor with oversized tires by greatly increasing the caster angle.
In the past, the practice was to use hydrostatic drive motors on the front wheels of tractors in combination with small tires steerable at large turning angles to make small radius turns. These hydrostatic front wheel drives are subject to spinout and are tractively inefficient.
In order to overcome the above deficiencies, agricultural manufacturers have become interested in mechanical front wheel drives which are less subject to these deficiencies. Based on European experience, it has been found that the front wheel tires must be approximately 30% larger while the preferred tread setting (or distance between tires) is a narrow 1,600 millimeters.
After experimentation with the larger tires and the preferred tread settings, it was determined that conventional steering provided excessively large turning radii because the inside turn wheel tends to interfere with the side of the tractor.
In order to reduce the turning radius of a mechanical front wheel drive tractor to that comparable with a conventional hydrostatic front wheel drive tractor, the steering geometry was analyzed and varied to determine the effect of such variables as axle length, axle pivot offset, kingpin angle, camber angle, oscillation angle, and turning angle.
After extensive analysis and computer simulation, it was determined that by increasing the caster angle to angles not heretofore used in any vehicle, that it was possible to reduce the turning radius of the tractor by causing the wheel to lie down and tuck underneath the body of the tractor.
In the past, as is well known to those skilled in the art, it was undesirable to exceed approximately 5.degree. caster angle because the steering force required to turn the wheels increases drastically with increased caster angle with no improvement in any other operating characteristic. However, analysis and experimental construction has shown that it is possible to have positive caster angles in the range of 11.degree. to 30.degree. which will allow the achieving of small turning radii without interference of the oversize wheels with the tractor body.